What do I do? I have a profile on a site, and this pervert just won't stop sending me messages saying he can guess everything about me and that he would love sex with me, and constantly comments on what he wants to do to me. Unfortunately, there is no way of blocking him. What the hell do I say/do?

Play him along. Get a load of pics off Google/FB/MySpace/Photobucket etc and make him believe they're you, and have a laugh with your mates about how pathetic he is. If you can get him to reciprocate by sending you obscene material then you might have a case against him. Whichever path you choose, just have fun with the idiot rather than stressing. Be damn sure he cannot access personal info or properly identify you or your location though.